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B777 Crew oxygen system

Old 5th Sep 2016, 22:37
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B777 Crew oxygen system

A question for anyone familiar with the servicing of the cockpit crew oxygen system on the B777.
Would a drop in pressure of some 730 psi (from fully charged at 1850 psi to 1120 psi) over about a 50 day period raise any concerns? Over the 50 day period the aircraft was used on 75 flights and accumulated some 500 hours.
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Old 5th Sep 2016, 23:17
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I know nothing about the 777 and little about all else* but I assume that the drivers test their masks as part of their pre flight checks and that little puff of O2 adds up. I am, in fact, surprised that the loss is so small.


* 36 years B2 though.
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Old 6th Sep 2016, 01:55
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Aircrew perform a 5 second check on their mask before each flight and the system itself is bled for 15 seconds at the start of each flight as part of the pressure check. So there's about 30 seconds of 100% oxygen bled off per flight.
730 psi represents about 40% of the system's capacity and, by my calculations, is 4-5 times more than what you'd get from the pre-flight and start up checks.
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Old 7th Sep 2016, 12:36
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In our airline (UK based) routine maintenance calls for replenishment of the crew oxygen system if the pressure falls below 1600 psi every daily chk. Other jurisdictions require bottle changes with no possibility of recharging in situ. If proper "defect coding" of the tech log entry related to servicing is carried out I'm sure a computer somewhere will flag up any unusual frequency, and alert tech services to investigate!
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Old 7th Sep 2016, 13:11
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Originally Posted by yotty View Post
In our airline (UK based) routine maintenance calls for replenishment of the crew oxygen system if the pressure falls below 1600 psi every daily chk. Other jurisdictions require bottle changes with no possibility of recharging in situ. If proper "defect coding" of the tech log entry related to servicing is carried out I'm sure a computer somewhere will flag up any unusual frequency, and alert tech services to investigate!
Thanks yotty.
I am looking at an investigation report that says "the decay in pressure from the nominal value of 1850 psi [to 1120 psi over 75 flights] was not unusual.". However, it strikes me as highly unusual. By my calculations, pre-flight mask testing and bleed on engine start up should have accounted for no more than a 200 psi drop over 75 flights, nowhere near the 730 psi drop recorded.
I don't have access to the maintenance records so I was hoping a B777 maintainer could say yea or nay, not unusual or unusual.
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Old 7th Sep 2016, 17:48
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not unusual at all. However we charge/change bottles as soon as they get below 1500psi!
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Old 7th Sep 2016, 22:03
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I wouldn't say that PSI loss is unusual over 75 days.

Lots of factors - flight crew may have played with the masks a bit more (ie: a line check or initial line indoc or something), some over-zealous engineers giving it a huff during cleaning? Tough to nail it down. Our guys are supposed to ask us to re-pack the masks in the containers if they pull them out (for training, for fun...what ever). Maybe some guys didn't. (Check your pilots' Facebook for pics with them wearing Ray-Bans with O2 mask trying to look cool ).

Perhaps mask may also be set to 'Emergency'? When they press the test-button on the container it may be spewing out O2 in the mask, instead of just showing the flow-eye-ball-thingy working? Not sure if it would do this though - would have to check.

Our practice is to service when <1600 PSI. Min-Dispatch for our varied types (for 2 crew) is 1300 PSI so gives us bit of leeway at the outstations. Always top off if jumpseating crew or relief-pilot on board.

I'd hate to work in a jurisdiction that doesn't let your service the bottle! Bizarre.
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Old 8th Sep 2016, 03:08
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I'd hate to work in a jurisdiction that doesn't let your service the bottle! Bizarre.
And a bloody OH&S pain - getting oxy bottles out of the E&E hatch on the Airbuses was an annoyance to say the least.

Mick - I wouldn't be too worried about a once off 'leak' like that. Strange things happen on long layovers, etc.... It's been 4 years since I was last near a 777, don't assume the preflight test is done exactly the same way by each crew. People I know (not me, I swear) used to sometimes need a refresh after a long day.... (Of course we had the O2 cart nearby to top back up!)
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Old 8th Sep 2016, 10:29
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spannersatcx, plhought, CAR42ZE, thank you for your insights, very much appreciated.

CAR42ZE, yes, I've heard about the long layovers and the, ahem, refreshers ... apparently sometimes at the start of a long day, particularly when preceded by a longer night.
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Old 8th Sep 2016, 11:19
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Also, it is a Transit Inspection requirement in some airlines for maintenance engineers to test mask operation before each flight.
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